The present claimed invention relates to the field of digital communication. Specifically, the present claimed invention relates to an apparatus and a method for managing multipath signals to be demodulated by a receiver with multiple demodulators.
Wireless telephony, e.g. mobile phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technologies. Because of increasing demand and limited resources, a need arises to improve their capacity, fidelity, and performance.
Referring to prior art FIG. 1A, an illustration of multipath signal propagation between a conventional base station and a mobile phone is shown. A conventional base station 104 transmits a signal to a mobile station, e.g., phone, 102. Typically, the signal contains pilot information, that identifies the base station, and data information, such as voice content. A signal that can be transmitted directly to mobile phone 102 without interference, such as first signal 106a, provides the strongest signal. However, given the power limitations at which base station 104 can transmit the signal, and given the noise a signal may pick up, a need arises to improve the power and the SNR of the signal received at mobile phone.
Conventional methods will combine the portions of the transmitted signal that travel different paths to mobile unit 102. The multiple paths arise because of natural and man-made obstructions, such as building 108, hill 110, and surface 112, that deflect the original signal. Because of the paths over which these other signals travel, a time delay and performance deterioration intrinsically arises in the synchronization-sensitive and noise-sensitive data transmitted from base station 104 to mobile phone 102. However, to provide the strongest possible signal to a mobile phone, two or more of the signals from these multiple paths, e.g. path 106a-106d, may be combined. However, to efficiently combine and demodulate multipath signals, a need arises for a method to select the most worthwhile candidates from all the different multipaths received in mobile phone.
Corruption of a transmitted signal falls into two general categories: slowly-varying channel impairment and fast fading variation. Slowly-varying channel impairment arises from factors such as log-normal fading, or shadowing caused by movement or blocking as exemplified in prior art FIG. 1A, or slow fading. Slower variations, e.g., sub Hz, determine in effect, the xe2x80x9cavailabilityxe2x80x9d of the channel. In contrast, only the fast fading variation affects the details of the received waveform structure and the interrelationships of errors within a message. Hence, a need arises for a method that effectively choose the properties of the signal that influence its condition for demodulation.
Referring now to prior art FIG. 1B, a graph of two conventional multipath signal strengths over time is shown. Graph 100b has an abscissa 122 of time and an ordinate of signal-to-noise ratio (SNR) 120, e.g. pilot Ec/Io ratio. Third multipath signal 106c and fourth multipath signal 106d are shown as exemplary multipath signals received at mobile phone 102. Conventional methods typically select for combining, the multipath signals with the highest SNR. Thus, at time span A 124a, the dark line representing fourth multipath signal 106d has a higher SNR level than third multipath signal 106c, assuming both signals have the same noise level. However, at time span B 124b, the dashed line representing third multipath signal 106c has a higher SNR level. Given the closeness of the SNR, or of the signal to noise ratio, of these two multipath signals, the choice as to which signal will be chosen for the demodulator can oscillate back and forth.
This oscillation is a condition known as xe2x80x9cthrashing.xe2x80x9d The drawback with thrashing is that it consumes a significant amount of system resources, such as processor operations. During thrashing, the processor can be overloaded with operations that constantly assign and deassign the multiple demodulators to different multipath signals. Furthermore, thrashing may degrade the quality of the mobile phone 102 output signal, as the switching may cause a loss of data or an audible interference or it may introduce latency effects. Consequently, a need arises for a method to select the best multipath signal for combining while avoiding the effect of thrashing.
In summary, an apparatus and a method are needed to improve the capacity, fidelity, and performance of digital communication. More specifically, a need arises for a method to improve the power and the SNR of the signal received at mobile phone. In particular, a need arises for a method to select the most worthwhile candidates from all the different multipaths received in mobile phone for a subsequent demodulation and combining operation. Additionally, a need arises for a method to select the best multipath signal for combining while avoiding the effect of thrashing.
The present invention provides a method and apparatus for improving the capacity, fidelity, and performance of digital communication. More specifically, the present invention provides a method that improves the power and the SNR of the signal received at mobile phone. Beneficially, the present invention provides a method to select the most worthwhile candidates from all the different multipaths received in mobile phone for a subsequent demodulation and combining operation. Additionally, the present invention implements this method without the detrimental effects of thrashing.
In one embodiment, the present invention recites a method comprising several steps. In the first step the multipath signals are received at a wireless communication device. Next, multipath signals are searched by a searcher portion of the wireless communication device. The multipath signals having sufficient signal strength for demodulation are required. Then the SNR level and the arrival time are determined for the one multipath signal acquired. The wireless communication device stores the information about an arrival time, an SNR level, and a transmitting base station of each multipath signal found previously in a multipath list wherein the multipath signals are categorized into a plurality of states. Once the arrival time and the SNR level of the acquired multipath are determined, the multipath list is updated with the arrival time and the signal-to-noise ratio if the acquired multipath signal is identified based upon the arrival time with one of the multipath signals existing in the multipath list. Subsequently, if the acquired multipath signal does not match any one of the multipath signals existing in the multipath list based upon the arrival time, the acquired multipath signal is considered to be newly-found and the newly-found multipath signal is categorized into one of a plurality of states based upon the signal-to-noise ratio. The multipath signals in the multipath list are moved from one to another of multiple possible states based upon search measurement results. Finally, the one multipath signal acquired is evaluated for a demodulation operation based upon its state.
These states in which an acquired multipath signal may be categorized include a temporary state, a potential state, and an assigned state. The temporary state is a state to which low quality signals are categorized, a potential state is a state to which medium quality signals are categorized, and an assigned state is a state to which high quality signals are assigned. The quality of the signal is defined in terms of whether they meet certain performance thresholds. The thresholds include both a power, or SNR, level, and/or a temporal threshold, e.g. period of time over which the power, or SNR, level is maintained. That is, signals categorized in the assigned state are chosen first for demodulation because they are the highest quality signal. If no signals are categorized in the assigned state, a queue of potential signals may be available for immediate assignment to a demodulator for demodulation, thus avoiding latency problems of the prior art. By using this practice, the present invention provides a queue of potential signals that are categorized in states according to historical performance. Thus the present invention overcomes the thrashing drawbacks of the prior art signal selection by using a state environment in which to categorize the multipath signals.
In another embodiment, the present invention recites a communication device having a transceiver, a searcher, a receiver, a processor and a computer readable memory, all coupled together. The memory portion of the communication device contains program instructions that, when executed via the processor, implement the aforementioned method for managing demodulators for multipath signals in a communication device.
These and other objects and advantages of the present invention will become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.